Difference between LMS/MRS level 1~2 are pretty minor however on level 3 there are noticable difference. There’s a article on nvidia’s website detailing the specific difference, generally LMS gets a slightly shaper image against MRS.
Meanwhile to be very honest the difference overall can be ignored under current HMS resolutions, but the difference became larger when looking in 4K HMDs.
VR SLI is tremulously useful at least for me. But the issue you can’t solve it by SinglePassStereo, VR SLI won’t do too much help as well (unless taking to Titan Xp).